1. Field of the Invention
This invention relates to a vibration wave motor which is driven by a traveling vibration wave.
2. Description of the Related Art
FIG. 1 shows a schematic diagram of a vibration wave motor (also termed an ultrasonic motor) which is driven by a traveling vibration wave, such a motor, which has recently seen widespread use, is described, for example, in U.S. Pat. Nos. 4,495,432, 4,513,219 and 4,779,018. In FIG. 1, a piezoelectric element 1 is used as an electromechanical energy conversion element, and made of, for example, PZT (lead zirconium titanate). A ring-like vibrator 2 is made of an elastic material, and many piezoelectric elements 1 are adhered to one surface of the vibrator 2. The vibrator 2 is held by a stator (not illustrated) together with the piezoelectric elements 1. A movable body 3 having an almost identical shape as that of the vibrator 2 is contacted to the vibrator 2 by means of pressure, and forms a rotor. The piezoelectric elements 1 are divided into two groups, one of which is disposed at a pitch shifted by 1/4 of the wavelength .lambda. of a vibration wave relative to another group. Each piezoelectric element within each group is disposed at a pitch corresponding to 1/2 of the wavelength, and so that polarities of adjacent elements are reversed relative to each other.
In a vibration wave motor having such a configuration, when a voltage is applied to the piezoelectric element 1 in the direction of its thickness (the direction of its polarization) by an AC power supply 9 as shown in FIG. 2, the piezoelectric element 1 expands and contracts in a direction perpendicular to the direction of the voltage application.
Suppose that an AC voltage applied to one group of the piezoelectric elements 1 is V.sub.0 sin .omega.T, and an Ac voltage applied to another group of the piezoelectric elements 1 is V.sub.0 cos .omega.T, where T is time and .omega.=2.pi./.lambda..
Since polarities of adjacent piezoelectric elements are reversed relative to each other and the AC voltages, which are shifted in phase by 90.degree. relative to each other, are applied to the two groups, respectively, the vibrator 2 produces a bending vibration in accordance with the pitch at which the piezoelectric elements 1 are disposed due to the expansion and contraction of the piezoelectric elements 1. That is, due to the relationship of the piezoelectric groups and the driving force on the group, when one group of piezoelectric elements of the vibrator 2 protrudes, the other group of piezoelectric elements retracts. Furthermore, since one group of the piezoelectric elements is displaced by 1/4 of the wavelength relative to the other group as described above, the bending vibration so produced advances as a function of time.
Consequently, vibration is successively excited while the AC voltage is applied, and is transmitted through the vibrator 2 as a traveling bending vibration wave.
FIGS. 3(a), 3(b), 3(c) and 3(d) indicate the relationship between the traveling states of the wave and the movable body 3.
Suppose that the above-described traveling bending vibration wave proceeds in the direction of the arrow X.sub.1. If the line O--O in FIG. 3 is assumed to be the central plane (neutral plane) of the vibrator 2 at a stationary state, the vibrator 2 deforms into the state indicated by the chain lines 6 in a vibrating state, and bending stresses at the the chain line (neutral plane) 6 in the vibrating state balance each other.
Relative to a cross sectional plane 7.sub.1 which is perpendicular to the neutral plane 6, the line of intersection 5.sub.1 (into the page) of these two planes is on the neutral plane 6 in variations in FIGS. 3(a) through 3(d). Hence, no stress is applied at the line of intersection 5.sub.1, which is only vibrating upwardly and downwardly.
On the other hand, the cross sectional plane 7.sub.1 performs a double-ended pendulum vibration to the right and left centering around the line of intersection 5.sub.1 (cross sections 7.sub.2 and 7.sub.3 also perform pendulum vibrations to the right and left centering around the lines of intersections 5.sub.2 and 5.sub.3, respectively).
Now, operation of the motor will be sequentially explained.
In the state shown in FIG. 3(a), a point P.sub.1 on the line of intersection between the cross section 7.sub.1 and the surface of the vibrator 2 at the side of the moving body 3 is at the right-most position of a right and left vibration, and only has a movement component in the upper direction. That is, in this pendulum vibration, a movement component in the left direction (a direction reverse to the proceeding direction X.sub.1 of the wave) is added when the line of intersection 5.sub.1 is at the positive side of the wave (at the side over the central plane 0--0), and a movement component in the right direction is added when the line of intersection 5.sub.1 is at the negative side of the wave (at the side under the central plane 0--0). The situation is the same for the lines of intersection 5.sub.2 and 5.sub.3. Namely, when the line of intersection 5.sub.2 and the cross section 7.sub.2 are in the former state as shown in FIG. 3(a), a movement component in the direction of the arrow (adjacent point P.sub.2) is added to a point P.sub.2. When the wave proceeds, and the line of intersection 5.sub.1 moves into the positive side of the wave as shown in FIG. 3(b), the point P.sub.1 moves to the left as well as in the upper direction. In FIG. 3(c), the point P.sub.1 only moves in the left at the upper dead center of the vertical vibration. In FIG. 3(d), the point P.sub.1 moves to the left and downward directions. The wave further proceeds, and returns to the initial state in FIG. 3(a) after a movement in the right and downward directions and a movement in the right and upward directions. Thus, the point P.sub.1 performs a spheroidal movement which synthesizes a series of movements as described above.
The movable body 3 is contacted to the vibrator 2 which performs such a spheroidal movement by means of pressure as described above. Considering the relationship between, for example, the movable body 3 and the point P.sub.1 on the vibrator 2 as shown in FIG. 3(c), it can be understood by the above explanation that the spheroidal movement frictionally drives the movable body 3 in the direction of X.sub.2. The points P.sub.2 and P.sub.3 and all other points on the vibrator 2 also frictionally drive the movable body 3 in the same manner as the point P.sub.1 does.
As described above, the vibration wave motor is essentially a motor which is driven by friction. Accordingly, the driving force of the vibration wave motor largely depends on the product .mu.W of a pressure W for contacting the movable body 3 to the vibrator 2 by means of pressure and a coefficient of friction .mu. between the movable body 3 and the vibrator 2. Consequently, it will be observed that the driving force is increased by increasing, for example, the pressure W. However, if the pressure W is too much increased, the bending vibration is suppressed, instead causing a decrease in the driving force. The movable body 3 may also be deformed due to the pressure and fall into a valley of the vibration wave. Points on vibrator 2 in a valley of the bending vibration wave move in a direction opposite to the apex of the vibration wave, as can be understood from FIG. 3. Hence, the driving force is largely reduced when movable body 3 is deformed into a valley. Accordingly, relative to the friction driving of the vibration wave motor, it is not recommended to increase the pressure W too much, and it is instead necessary to combine materials having large coefficients of friction .mu. in order to increase the driving force.
In general, however, the friction between materials having large coefficients of friction .mu. becomes very large. Hence, there is the disadvantage that the life of the vibration wave motor becomes extremely short when materials of such a combination are used in the vibration wave motor.